Communication systems, such as Code Division Multiple Access (CDMA) systems, communicate messages between infrastructure equipment and mobile units. As used herein, a forward message refers to a message generated by cellular infrastructure equipment and transmitted for reception by a mobile communication unit, and a reverse message refers to a message generated by a mobile communication unit, such as a mobile cellular phone.
Currently, two methods are used by CDMA systems to ensure accurate delivery of an important forward traffic channel message to a mobile unit. One example of an important forward traffic channel message is an Extended Handoff Direction message, which directs the mobile to hand off to another base station. The first method is an L2 acknowledgment (Ack) mechanism, and the second method is a quick repeat mechanism.
For the L2 Ack method, per the CDMA air interface standards (IS-95 and J-STD-008), the mobile unit is required to send back an L2 Ack indication within 200 milliseconds from the time the forward traffic channel message is successfully received. The L2 Ack indication, if received by the infrastructure, explicitly notifies the infrastructure that the forward traffic channel message has been received by the mobile unit. If the L2 Ack indication is not received by the infrastructure within a predetermined time limit, the infrastructure may resend the message. The L2 Ack indication may not be received by the infrastructure due to the mobile unit never receiving the forward traffic channel message and therefore has not sent back an L2 Ack indication, or the mobile unit receiving the forward traffic channel message and sending back the L2 Ack indication that is never received by the infrastructure.
One advantage of the L2 Ack method is that the infrastructure will know fairly reliably whether the mobile unit has received the forward traffic channel message. One disadvantage, however, is that there can be a long delay before the infrastructure knows whether the forward traffic channel message has been accurately delivered. When the mobile unit has not received the forward traffic channel message or the infrastructure did not receive the L2 Ack indication, the infrastructure will wait until the timer associated with the L2 Ack mechanism expires to realize that the forward traffic channel message needs to be resent.
A second method for delivering a forward traffic channel message is to use quick repeat techniques. The quick repeat technique sends the same forward traffic channel message multiple times within a certain time frame. This repetition leads to an increase in the possibility that the mobile unit will receive the forward traffic channel message. Although the success rate should be very high, one disadvantage is that the infrastructure will not explicitly know if the forward traffic channel message has been successfully delivered to the mobile unit. Further, the infrastructure may send out a message multiple times unnecessarily, if the mobile unit successfully received the message on the first transmission. One advantage, however, is that there is a decrease in reverse messages sent from the mobile unit to the infrastructure.
Consequently, a need exists for a method for determining a need to retransmit a message that does not require the cellular infrastructure equipment to unnecessarily transmit multiple forward messages, while not increasing the number of reverse messages transmitted by mobile units.